They turned out more than 1,000 washing machines, most of which _______ quality.

　 A. is good　　　 B. are good　　　　 C. is of high　　　 D. are of high

The government has taken measures 　　　　 air quality.

　 A．to improve　 B．improved　　 C．being improved D．to be improved

— The bike has broken down many times since I bought it.

— Well, they _____ have paid more attention to quality.

　 A. must　　　 B. can　　 C. may　　 D. should

_______ different kinds of pianos, the workers farther improved their quality.

　 A. To produce　   B. Being produced　      C. Produced         D. Having produced

. _______ different kinds of pianos, the workers farther improved their quality.

　A. To produce　   B. Being produced　      C. Produced         D. Having produced